Currently, the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM). Further description of the GSM TDMA communication system can be found in ‘The GSM System for Mobile Communications’ by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.
To further enhance the services and performance of the GSM communication system, a number of enhancements and additions have been introduced to the GSM communication system over the years.
One such enhancement is the General Packet Radio System (GPRS), which is a system developed for enabling packet data based communication in a GSM communication system. Thus, the GPRS system is compatible with the GSM (voice) system and provides a number of additional services including provision of packet data communication, which augments and complements the circuit switched communication of a traditional communication system. Furthermore, the packet based data communication may also support packet based speech services. The GPRS system has been standardised as an add-on to an existing GSM communication system, and can be introduced to an existing GSM communication system by introducing new network elements. Specifically, a number of Serving GPRS Support Nodes (SGSN) and Gateway GPRS Support Nodes (GGSN) may be introduced to provide a packet based fixed network communication.
3rd generation systems are currently being rolled out to further enhance the communication services provided to mobile users. One such system is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed. Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876. The core network of UMTS is built on the use of SGSNs and GGSNs thereby providing commonality with GPRS.
In order to further enhance the services and functionality that can be provided by cellular communication systems, standardisation work has already begun on improvements and enhancements for the 3rd Generation cellular communication systems. Specifically, a work item known as EUTRA (Enhanced UMTS Terrestrial Radio Access) is currently defined for investigating new cellular techniques for future enhancement of the UMTS Terrestrial Radio Air-interface.
3rd Generation cellular communication systems tend to use a technique called soft handover wherein simultaneous links are maintained between a user equipment and a plurality of base stations during a handover. However, for EUTRA, Fast Cell Selection (FCS) is preferred over soft handover for the downlink direction. Fast cell selection is a technique wherein the user equipment continuously monitors the quality of signals received from the serving base station and one or more neighbour base stations. The signals are used to evaluate which base station is currently the most suitable serving base station. The system incorporates a fast signaling mechanism which allows the user equipment to be quickly switched between base stations. Thus, in fast cell selection, the user equipment controls fast hard handovers between different base stations thereby allowing the communication to be supported by the most suitable base station.
In EUTRA, Orthogonal Frequency Domain Multiple Access (OFDMA) is one promising candidate for the downlink access technique. In contrast to CDMA systems, implementing soft handover for OFDMA is likely to require non-overlapping time and frequency allocations from the multiple base stations during the soft handover. However, this is wasteful in terms of the scarce radio resource and therefore fast cell selection promises to be a more suitable solution. Furthermore, for EUTRA fast scheduling of user equipments will be employed in the base station which does not lend itself to soft handover since it requires complex and impractical coordination between the scheduling performed by different base stations.
However, fast cell selection also has some associated problems and disadvantages. For example, signaling errors may occur which can result in the selected base station not receiving the cell reselection message resulting in the call being interrupted or dropped.
Furthermore, it is difficult to maintain the continuity of data when switching between the different cells without introducing complex algorithms and duplicated communication and storage of data at the involved base stations. However, this requires increased functionality, storage capacity and increased bandwidth of the communication links in the fixed network. Accordingly, disruptions can frequently occur when switching between cells and/or a significant delay can occur.
For example, for a system using a retransmission scheme, it is necessary to either wait for all data that has been forwarded to the previous serving base station (from for example the core network; a base station controller; or another base station) to be successfully acknowledged by the user equipment or it is necessary to forward duplicated data to the new base station. Thus, re-transmissions may be pending at the time of the cell change which must either be completed (adding latency) or aborted (resulting in waste of transmission resources). Accordingly, either an additional delay or increased bandwidth consumption results. Furthermore, in order to have an efficient switchover, the synchronization of the data buffers at the multiple base station is also required i.e. the newly selected base station needs to know which data has been successfully transmitted to the user equipment. This introduces additional complexity.
Hence, an improved system would be advantageous and in particular a system allowing increased flexibility, reduced delay, reduced bandwidth consumption, facilitated operation, reduced complexity and/or improved handover performance would be advantageous.